I agree that would help, but from working from the other side of the coin, running a small garage, there are two main issues.
1. It is like going to a restaurant and bringing your own food as given the high overheads of running a business part of your income is the margin on parts, which if good quality parts may only be 10% but makes a difference.
2. Often the customer can supply either the wrong part or an inferior part which can block the workshop up whilst waiting for resolution. Where as should the trade supplier send the wrong part they will usually rectify on the next delivery van within the hour.
If you are able to do your own repairs then none of this is a problem if waiting for parts, but for a business it can be a significant loss of income, especially if paying a mechanics wage to stand around